MMP-9 affects dural cell composition

The gelatinase, matrix metalloproteinase (MMP)-2 and MMP-9, fine tune inflammatory processes that facilitate immune cell penetration of the parenchymal (astroglial) border at the blood-brain barrier (BBB) during neuroinflammation. Whether they are expressed in the dura or contribute to immune regulation at this site, an additional brain border tissue that has recently gained considerable attention, remains unknown. Here, we show that both pro- and activated-MMP-2 and MMP-9 are present in the dura of naïve wild-type (WT) mice, with selective upregulation of MMP-9 during experimental autoimmune encephalomyelitis (EAE). Mmp9-deficient mice displayed delayed EAE onset but developed more severe disease at peak, accompanied by a marked accumulation of Ly6G? cells in the dura. Single-nucleus RNA-seq of naïve dura revealed Mmp9 expression predominantly in myeloid populations. Comparative analyses of EAE dura from WT and Mmp9-/- mice identified a venous endothelial cell subset with the most pronounced transcriptional alterations, alongside an expanded granulocyte compartment enriched for mature neutrophil signatures, in the knockout condition. Flow cytometry confirmed increased numbers of Ly6C+Ly6G+ neutrophils in the dura of Mmp9-/- mice during EAE compared to WT controls. Our data suggest that MMP-9 modulates dural immune responses during EAE, potentially through the regulation of both the inflammatory microenvironment and specific immune cell recruitment. While Mmp9 deficiency delays EAE onset, it appears to promote neutrophil accumulation and inflammatory responses in the dura, suggesting an anti-inflammatory role for MMP-9 at this CNS border, which contrasts with its pro-inflammatory function at the BBB